Over-temperature detection is important for the protection of personnel and equipment. Electronic equipment will fail if allowed to exceed maximum temperature levels. Incipient mechanical equipment failure can be detected by sensing overheating and shutting down the equipment. Agricultural products, ammunition, power cables, etc. must all be stored or operated below maximum temperature levels to avoid damage.
Over-temperature at a single point is conventionally sensed by a bimetallic thermostat which is attached to the component to be protected and which is connected to a power supply. Such a thermostat is normally open and when the temperature of the component rises above the preset limit, the thermostat closes, connecting the power supply to an alarm, an indicator, or a control relay. For multiple point sensing, one power supply can be used for multiple thermostats. However, the primary difficulty with using discrete detectors such as thermostats is that the hot spot can occur between the detectors and either go unnoticed or be detected after the damage has been done.
The parameters commonly employed for continuous over-temperature sensing include pressure and resistance.
In pneumatic devices, i.e. those employing pressure sensing, a tube is pressurized with an inert gas. The application of heat causes an increase in pressure which operates a diaphragm that closes an electrical contact. Upon removing the heat, the pressure falls and the contact reopens.
Resistance sensors rely on a decrease in resistance at the "hot-spot" between two conductors separated by a dielectric. Meltable plastic devices utilize a twisted pair of wires, each wire encased in meltable plastic material. When the temperature exceeds the material's melting point, the wires touch. To reuse the sensor, the shorted section must be cut out and replaced. A continous thermistor cable comprises two conductors separated by continuous negative temperature coefficient of resistivity material. As the temperature of such cable rises, the resistance between the two conductors fails. Once the temperature falls, the resistance returns to its original high value. Eutectic salt type devices utilize a salt compound between two conductors. At the eutectic temperature, the compound melts and its resistance falls, connecting the conductors. When the temperature falls, the compound solidifies and the resistance returns to its original high value.
Each of the above discussed technologies place fundamental limitations on the construction and performance of continuus over-temperature sensors which do not exist in the present invention which utilizes conductive polymer technology. Specifically, the pneumatic tube devices are inherently incapable of determining hot spot location, since they sense only an increase in pressure within the tube. The meltable plastic material can only be used once and must then be repaired, and is, therefore, unable to serve as long lived monitor. The thermistor and eutectic salt devices require a metal casing for the sensor making it relatively inflexible, subject to corrosion, difficult to splice and expensive to manufacture.
Another parameter which is useful for temperature-sensitive continuous monitors is capacitance. Here, the increase in resistance of one conductor at the anomaly point--a hot or cold spot along the cable--varies the effective capacitance between that conductor and another, normally temperature-insensitive, conductor. Such a system is disclosed in U.S. Pat. No. 4,041,771, which discloses a temperature sensitive detector that senses capacitance changes in an elongated container which holds a body of material which changes phases at specific temperatures. Specifically, said patent discloses an inflexible tube containing a liquid which changes conductivity upon freezing and thereby changes the capacitance of the entire device. The use of a liquid to a solid phase-changing material limits such a device to only the structure disclosed, namely a liquid within a dielectric tube surrounded by a conductor. Furthermore, the teaching of a liquid type device would appear to be limited to the sensing of an under-temperature condition, since it is the increase in resistance which changes the capacitance in a measurable manner, by effectively changing the size of one of the plates of the capacitor.
The instant invention is an improvement over the above described device, said improvement comprising a flexible cable having (1) an electrical conductor, (2) a dielectric, and (3) a conductive positive temperature coefficient of resistance (hereinafter referred to as PTCR) polymeric composition in a geometric relationship such that the dielectric separates the electrical conductor from the conductive polymeric composition. Because the polymeric composition possesses a positive temperature coefficient of resistance, the cable acts as an over-temperature sensor, rather than the under-temperature sensor described in the previous paragraph. The instant invention also provides for a construction in which the temperature sensitive conductive polymer can be placed adjacent to the mass whose temperature is being monitored, i.e., people or equipment, thereby increasing sensitivity and response time of the total device.
The instant invention is a significant improvement over the prior art for three basic reasons. First, the instant invention utilizes a solid temperature sensitive material in the form of a conductive PTCR polymeric composition which is necessary for construction of practical reusable cables possessing flexibility, ruggedness and durability. Second, the use of solid temperature sensitive material makes possible construction of an over-temperature sensor. Third, and most importantly, the instant invention provides a massive resistance increase at this over-temperature location which when combined with a capacitance measuring technique, makes it possible to locate the over-temperature position with great accuracy. When the critial sensing temperature is exceeded in the solid conductive polymeric composition of the instant invention, the resistance increase can be four or more orders of magnitude over a small temperature range (e.g. 5.degree. to 10.degree. C.). The large resistance change is necessary to electrically isolate the portion of the cable beyond the hot spot so that it does not contribute to the capacitance of the cable between the control end and the hot spot, thus making precise location of the hot spot possible. The sharp change in resistance at the critical temperature facilitates construction of a monitor having adequate temperature resolution. The resistance increase only comes with falling temperature if the liquid to solid transition is used, and therefore, liquid to solid transition is of no value whatsoever in over-temperature sensing.